In certain applications, a switch is required to connect two or more nodes together where these nodes may assume a wide range of voltage levels. For example, with reference to FIG. 1, if switch SW1 is being used to intermittently connect current pump output terminal 20 to input terminal 22 of load 24, the voltage on driving terminal 20 or input terminal 22 may vary significantly between the power supply voltage and ground potential. Switch SW1 must be able to remain on or off over this wide range of voltages applied to its terminals.
If switch SW1 in FIG. 1 is a MOSFET with an insulated gate, shown as MOSFET 28 in FIG. 2, it is fairly simple to ensure the MOSFET 28 remains fully on or off, despite variations in voltage at terminals 20 and 22, by applying either the full power supply voltage or ground potential to its gate.
However, for switching devices which do not use an insulating gate, such as bipolar transistors or MESFETs, the base or gate of the pass transistor must not be driven to a level which causes the transistor's inherent diodes to conduct a significant amount of current. Therefore, the full power supply voltage cannot be simply applied to the base or noninsulated gate of the pass transistor for turning the transistor on.
Further, when turning the pass transistor off, it may not be desirable simply to apply ground potential to its base or gate, since the transistor may be adequately turned off by using a voltage higher than ground potential. By applying ground potential to the transistor's base or gate, unnecessary delay may be incurred when turning the transistor back on.
Thus, it would be desirable to provide a FET or bipolar switching device which will turn on and remain on despite variations in voltages at its nodes and which will not allow inherent diodes in the transitor to be forward biased. Further, it may be desirable to provide a FET or bipolar switching device which will turn off and remain off without applying an unnecessarily low voltage to its gate or base so as to avoid any unnecessary delay when the transistor is to be turned on.
Still further, it may be desirable to provide a switching device which will maintain a zero voltage differential between two nodes when the switch is open to prevent any current spikes resulting from a non-zero voltage differential between the two nodes when the switch is subsequently closed.